Mac's Radio Service Shop: Intermittents Still PursueFebruary 1949 Radio & Television
News Article

Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.

Mac McGregor, owner of Mac's Radio Service Shop, can always be counted
on to provide his apprentice technician, Barney, with a lesson from
his own life-long attendance at the School of Hard Knocks. Barney
is your stereotypical young buck whose level of seriousness needs
occasional alignment, just as do the radio and television sets he
services. In this episode, I can't find where Mac actually solved
the intermittent electrical condition believed to be causing the
problem - weird. The "Mac's Radio Service Shop" series ran in
Radio &
Television News magazine for many years prior to
a similar electronics story series called
"Carl &
Jerry" appeared in
Popular Electronics. Both were created by consummate storyteller
John T. Frye.

Intermittents Still Pursue

By John T. Frye

When
Barney, the red-headed apprentice of Mac's Radio Service Shop, had
gone into the chassis-cleaning booth twenty minutes before, there
had not been a single wire recorder in the shop; but when he came
out, Mac was busily engaged in taking one out of its case.

"Why, the beautiful creature that persuaded you to run her wire
recorder around all of these other sets stacked up here! That gal
really must have had something."

"For your information, Junior, this wire recorder belongs to
the young minister of that church on Ninth Street. I am giving it
priority because of the use he puts it to. Each Sunday he makes
a recording of the entire church service. Then, during the week,
he takes it around to the homes of the sick and aged members who
cannot get out and lets them go to church right in their homes.
Several of these old people are quite hard of hearing, and he wants
me to install a phone jack for their benefit."

"It certainly is," Mac agreed, "and speaking of 'using the old
noodle,' as you vulgarly put it, I think it is about time we had
another chalk-talk on intermittents."

With a flourish Barney whipped out his beat-up notebook and perched
himself on the end of the bench. "Yes, sir; whenever you are ready,
sir," he invited with mock deference.

"Well," Mac began, still working on the recorder and pausing
now and then when what he was doing required a little extra attention,
"first I want to impress on your alleged mind that the amount of
time that elapses between the time the set is turned on and the
time at which the erratic behavior starts is very informative. Most
intermittent conditions are caused by expansion of defective parts;
and this expansion, in turn, is caused by heating. Since not all
of the parts warm up at the same time, but, instead; take on heat
according to a definite sequence, you can make a pretty shrewd guess
at where the defective part is by how long it takes for the trouble
to start."

"For example, the tubes heat up first; then the resistors increase
in temperature; gradually the heat spreads through the chassis and
warms up the coils and condensers. The proximity of a particular
part to a tube or hot resistor must also be taken into account.
In general, though, if your trouble begins as soon as the set is
turned on, be especially suspicious of tubes and resistors. The
filament of an a.c.-d.c. tube that opens up as soon as the filament
is hot and then reestablishes contact when it cools down is a well-known
example of this. Usually, but not always, this defect will show
up in the first five minutes of operation.

"Another case that is fairly common is that of the set that has
a lot of noise and abrupt changes of volume when it is first turned
on but that settles down and plays faultlessly after about fifteen
minutes of operation. In several cases, I have found the trouble
to be in a metal-encased resistor riveted to the chassis. The taps
on this resistor make poor contact when the resistor is cold; but
when the resistor heats and expands, it wedges the metal band of
the tap tightly between the element and the case and makes a good
contact until the resistor is allowed to cool down again ..

"If the set plays all right for a half-hour or so and then begins
to misbehave, it is a good idea to suspect condensers or coils.
We talked before about how to locate a condenser that is opening
up, but I want to mention, too, that you can often tell in what
circuit a condenser is opening by listening to the change in the
quality of the signal when it drops in volume. An opening coupling
condenser will often cause a severe loss of low frequencies, because
the highs will still be passed by the small amount of capacity between
leads, etc. A plate, cathode, or a.v.c. bypass that is opening will
often cause a characteristic hissing 'near-oscillation' sound to
accompany the drop in volume. A defective coupling condenser in
the oscillator circuit will often have a detuning effect on the
signal just before the set quits playing."

"You told me last month that changes in volume were not the only
troubles that came under the heading of 'intermittents,' " Barney
said. "What other kinds are there?"

"Well, intermittent noisy conditions are quite common. We already
have spoken about noisy resistors. Coils that open up are very common,
especially in circuits that carry d.c. current. This takes in the
.primaries of r.f., i.f., oscillator coils, and audio transformers,
and it also includes speaker field windings. An experi­\enced serviceman
can usually spot one of these 'opening coil' cases by the distinctive
sound they make. In addition to an intermittent rustling sound,
there is often a kind of high-pitched squeaking sound, like this:"

Mac drew his breath in between tightly-pursed lips to produce
the sound that many people use to call a near-by dog. Barney dropped
his notebook, clapped both hands over his heart, and rolled his
eyes blissfully toward the ceiling. "Ah!" he exclaimed rapturously,
"that reminds me of how Margie says goodnight:"

"You keep your mind on what. I'm saying, or I'll 'goodnight'
you," Mac warned trying to scowl fiercely. "You can often show up
which winding is going out by increasing the current through it.
To do this, connect a resistor of around 5000 ohms between the point
fed through the coil and the ground for two or three seconds. If
the coil is OK, it will pass this temporary overload without harm;
but if it is defective, the noise will become much worse or the
coil will open up completely.

"Another very common noisy condition, especially with a.c.-d.c.
sets, is caused by a defective filter condenser. I am not sure as
to exactly what happens inside the condenser, but the effect is
a loud scratching sound that may or may not be accompanied by a
noticeable increase in hum. Usually, if you bridge the defective
condenser with a good one, the surge that takes place will cause
the noise to stop abruptly; and ordinarily the noise will not start
immediately when the good condenser is removed. Sometimes it will
not commence again for several days. Often this noise will be radiated
and can be picked up by other sets in the same room, which will
fool you if you leave the defective set on while you try another
set to see if it is noisy, too.

"If you suspect this condition, turn the set off and clip a good
condenser across the suspected unit; then, after the set is playing,
gently remove the good condenser and see if the noise begins. In
this way, you will not 'cover up' the condition you are trying to
locate - as you are almost certain to do if you employ the usual
method of bridging filter condensers while the set is playing.

"Another very common noisy condition with a.c.-d.c. sets is caused
by noisy rectifier tubes such as the 35Z5's, 35Z3's, and 35Y4's.
Something happens inside these tubes so that an annoying scratching
sound is heard every time they are jarred ever so slightly. This
sound is not present when the volume is turned off or when the r.f.
and i.f. tubes are disabled; so it must be picked up by the antenna.
In fact, I have noticed that the noise is much worse in sets in
which the loop antenna is near the rectifier tube. Just the vibration
caused by the sound from the speaker will make the rectifier give
forth with this annoying sound. The cure, of course, is a new tube."

By this time Mac had the phone jack installed, and he began to
replace the recorder in its cabinet.

"An intermittent hum," he went on, "naturally causes you to suspect
the filter condensers first, and that is right; but the filter condensers
are not always at fault. Cathodes that develop partial or complete
shorts to the filaments as the tubes reach a certain critical temperature
are fairly common, especially in the a.c.-d.c. sets. A grid that
is left floating by a defective coil or resistor will introduce
a hum that can be spotted by observing that as the hand is brought
near the floating grid the hum will increase greatly. A hard-to-locate
hum will occasionally show up in a.c.-d.c. sets that do not normally
connect one side of the line to the chassis if the line becomes
shorted to the chassis, say through poor insulation in a dial-lamp
socket."

Mac paused briefly to tryout a pair of phones in the new jack.
Then he continued:

"Finally we come to those sets that start playing quite well
at first but gradually develop a progressively worse distortion.
The first thing to sus­pect is a leaky coupling condenser that is
lowering the bias on an audio tube. The leakage of a condenser is
often dependent on its temperature, and that is why it may take
some time for the distortion to show up. Measuring the grid bias
with a v.t.v.m. is the quickest way to check on a leaky coupling
condenser. There are times, though, when you can cut the coupling
condenser entirely loose from the grid and the grid will still read
positive. What is more, no positive voltage will be found at the
cold end of the coupling condenser. In that case, you have a tube
that is suffering from 'secondary emission.' Such a tube will gradually
draw more and more plate current and will cause more and more distortion.
If the grid resistor of such a tube is discovered to be at its rated
value, the only thing to do is to replace the tube. A too-high grid
resistor will aggravate or even cause this condition."

Mac paused, and Barney broke in hopefully:

"Is that all there is to know about intermittents?"

"Not by a long shot!" Mac said. "We have just hit the high spots
of what I know on the subject, and I am still learning something
new about intermittents nearly every day."

Barney heaved a big sigh as he put away his notebook and slid
from the bench. "The nice thing about you, Mr. McGregor, is that
you make radio servicing sound so-o-o-o easy!'; he said bitterly.

Mac's Radio Service Shop Episodes on RF Cafe

This series of instructive stories was the brainchild of none other than John T.
Frye, creator of the Carl and Jerry series that ran in Popular
Electronics for many years. Mac's Radio Service Shop began life in Radio & Television News magazine
(which itself started as simply Radio News), and then changed its name to Mac's
Service Shop after the magazine became Electronics
World. 'Mac' is electronics repair shop owner Mac McGregor, and Barney is his
eager, if not somewhat naive, technician assistant. 'Lessons' are taught in story format
with dialogs between Mac and Barney.

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas
and reference material while performing my work as an RF system and circuit design engineer.
The Internet was still largely an unknown entity at the time and not much was available
in the form of WYSIWYG
...

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used on the RF Cafe website are hereby acknowledged.